The invention relates to a semiconductor device for generating an electron current, comprising a cathode having a semiconductor body with an n-type surface region and a p-type region in which electrons leaving the semiconductor body can be generated in said body by giving the n-type surface region a positive bias with respect to the p-type region.
The invention also relates to a pick-up tube and a display device provided with such a semiconductor device.
Semiconductor devices of the type described above are known from Netherlands Patent Application No. 7905470 which corresponds to U.S. Pat. No. 4,370,797.
They are used, inter alia, in cathode ray tubes in which they replace the conventional thermionic cathode in which electron emission is generated by heating. In addition they are used in, for example, apparatus for electron microscopy. In addition to the high energy consumption for the purpose of heating, thermionic cathodes have the drawback that they are not immediately ready for operation because they have to be heated sufficiently before emission occurs. Moreover, the cathode material is lost in the long run due to evaporation, so that these cathodes have a limited lifetime.
In order to avoid the heating source which is troublesome in practice and also to mitigate the other drawbacks, research has been done in the field of cold cathodes.
The cold cathodes known from the aforementioned patent application are based on the emission of electrons from the semiconductor body when a pn-junction is operated in the reverse direction in such a manner that avalanche multiplication occurs. Some electrons may then obtain as much kenetic energy as is required to exceed the electron work function; these electrons are then liberated on the surface and thus supply an electron current.
In this type of cathode the aim is to have maximum possible efficiency, which can be achieved by a minimum possible work function for the electrons. The latter is realized, for example, by providing a layer of material on the surface of the cathode, which decreases the work function. Cesium is preferably used for this purpose because it produces a maximum decrease of the electron work function.
However, the use of cesium may have drawbacks. Inter alia, cesium is very sensitive to the presence (in its ambiance) of oxidizing gases (water vapor, oxygen, CO.sub.2). Moreover, cesium is fairly volatile which may be detrimental in those uses in which substrates or compounds are present in the vicinity of the cathode such as may be the case, for example, in electron lithography or electron microscopy. The evaporated cesium may then precipitate on the these objects.